Chairman Ferdinand Piech, who was forced to back CEO Martin Winterkorn after being isolated in a five-to-one board vote last week, asked Porsche CEO Matthias Mueller this week to be ready to replace Winterkorn, Der Spiegel reported on Friday.

The German magazine did not give the source of its information. Both Piech's office in Salzburg and Porsche declined to comment on Friday.

The leadership row burst into the open this month when Der Spiegel quoted Volkswagen patriarch Piech as saying he had "distanced" himself from Winterkorn.

The comment comes at a time VW is cutting billions of euros of costs and revamping structures to boost profitability and it prompted the vote by the six-member panel on VW's board that includes Piech and one Porsche family member.

However, a source close to VW said on Friday that the Piech and Porsche families, who together hold 51 percent of the voting shares in Volkswagen, had held another meeting this week, indicating that there are still ongoing differences.

Members of VW's 20-seat supervisory board are rapidly losing patience with Piech's behind-the-scenes machinations, which is hurting VW as it strives to tackle underperformance in foreign markets, sources said.

"The chairman has unilaterally plunged the company into the greatest turbulence possible," a labor representative on the board told Reuters. "He really should think about what that means for him and his own position."

Piech, a dominant figure at VW for more than two decades and the grandson of the inventor of the VW Beetle, would have faced calls for his own resignation last week had he not backed down to support Winterkorn, sources have said.

A person close to the state of Lower Saxony, VW's second-largest shareholder with 20 percent of the voting rights, also said stakeholders were losing patience with Piech's maneuvering and could try and put a stop to it.

"Two or three more things like this and that could be it," said the person close to the state of Lower Saxony.

Lower Saxony could tip the scales in any ballot on the board where Piech still holds a casting vote. Appointing or dismissing executives requires a simple majority while a possible vote to force out the chairman would need 14 of 20 votes.

VW employs more than 120,000 people in Lower Saxony, which has repeatedly called for consensual leadership.

Together with the powerful works council which occupies half the 20 board seats at VW, Lower Saxony has most outspokenly backed Winterkorn. Under the CEO's eight-year reign, VW has more than doubled sales and almost tripled profit.

(Additional reporting by Ilona Wissenbach; editing by David Clarke)

By Andreas Cremer and Jan Schwartz